A SAW filter utilizing a surface acoustic wave (SAW) has hitherto been in wide use as a frequency selective filter (hereafter also referred to simply as “filter”) for use in RF (radio frequency) stage of a mobile communications apparatus such as a cellular phone and an automobile telephone. In general, various characteristics that are required of a frequency selective filter include wide pass band, low loss, and high attenuation.
For example, for the broadening of SAW filter passband range, a SAW filter is used that has a longitudinally coupled resonator-type surface acoustic wave device in which a plurality of IDT (InterDigital Transducer) electrodes are arranged along a surface acoustic wave propagation direction and a reflector electrode is disposed on both sides of the whole of the IDT electrodes.
In order to achieve impedance matching within the range of the pass band of the SAW filter, especially in the case of a wide passband SAW filter, a matching circuit is commonly used. In FIG. 17, there is shown an ideal circuit configuration of a conventional SAW filter 20a having a matching circuit (Conventional example 1). For the impedance matching within the range of the pass band of the SAW filter 20a, a first signal line 9 connected to an input signal terminal 12 in the SAW filter 20a is provided with a first capacitor 35 in series with respect to a surface acoustic wave device 1 and a first inductor 36 in parallel with respect to the surface acoustic wave device 1, and a second signal line 10 connected to an output signal terminal 13 in the SAW filter 20a is provided with a second capacitor 37 in series with respect to the surface acoustic wave device 1 and a second inductor 38 in parallel with respect to the surface acoustic wave device 1. The first inductor 36 and the second inductor 38 are connected to a ground electrode 8.
In FIG. 18, there is shown the result of simulation as to the electrical characteristics (frequency-transmission characteristic relationship) of the SAW filter 20a having the circuit configuration shown in FIG. 17. As electrical characteristics, the SAW filter 20a exhibits wide passband capability with a bandwidth of approximately 100 MHz and is able to effect impedance matching with less ripples.
However, in the SAW filter in a condition of actual service, an ideal circuit configuration such as shown in FIG. 17 cannot be realized. The SAW filter generally operates in a state of being mounted on a dielectric substrate by means of solder. In this case, since wiring, through hole conductors, and so forth are present between a ground electrode of the dielectric substrate and the ground electrode 8 required for electrical evaluation, it follows that an inductance component inevitably arises due to the wiring, through hole conductors, etc. Therefore, in the SAW filter in a condition of actual service, the first inductor 36 and the second inductor 38 are connected to the ground electrode 8 via a parasitic inductance of the dielectric substrate. Due to the presence of such a parasitic inductance, the SAW filter 20a shown in FIG. 17 exhibits in reality an electrical characteristic as shown in FIG. 20 that, in contrast to the electrical characteristic of the ideal circuit configuration shown in FIG. 18, shows a sign of deterioration in attenuation outside the pass band. This is because part of electric current which flowed from the first signal line 9 to the first inductor 36 does not flow into the ground electrode 8 due to the parasitic inductance of the dielectric substrate but flows toward the second inductor 38, which leads to current leakage to the second signal line 10.